1. Field of the Invention
The present invention relates generally to antennas provided within a wireless local area network, and more particularly to a wireless local area network antenna and wireless local area network card implemented using the same, which can transmit/receive RF signals in a high frequency band (5 GHz) and a low frequency band (2.4 GHz) without increasing the size of the antenna, and simply adjust antenna characteristics without varying the structure of the antenna.
2. Description of the Related Art
Recently, with the miniaturization and weight reduction of mobile communication devices, and the multiplexing of a transmission/reception band to two or more bands, an antenna, one of important parts for the wireless transmission/reception of a mobile communication terminal, has been developed to an F or inverted F-type antenna from an external helical antenna.
Especially, in the case of a wireless Local Area Network (LAN), a dual band antenna capable of transmitting/receiving data in a 5 GHz frequency band as well as a currently used 2.4 GHz frequency band is required to enable large capacity data, such as multimedia data, to be transmitted afterward.
FIG. 1 is a view showing a conventional dual band antenna. As shown in FIG. 1, an antenna 11 comprises a radiation electrode 13 with a predetermined area, a slot 14 positioned in the radiation electrode 13 to multiplex a current path of the radiation electrode 13, a feeding electrode 16 for applying a current to the radiation electrode 13, and a ground electrode 15 for grounding the radiation electrode 13.
In FIG. 1, one slot 14 forms two current paths connected in parallel on the radiation electrode 13 on the basis of the feeding electrode 16, thus causing resonance to occur in two frequency bands corresponding to the respective current paths. Further, the two frequency bands in which resonance occurs are the transmission/reception bands of a corresponding antenna. Therefore, the two transmission/reception bands are determined by the areas of two radiation regions divided by the slot 14 of the radiation electrode 13.
The antenna shown in FIG. 1 is called a Planar Inverted F-type antenna (PIFA) according to the shape thereof. Besides the PIFA, a monopole-type antenna, having no ground electrode in the structure of FIG. 1, is also used.
However, if the conventional dual band antenna as shown in FIG. 1 is applied to a wireless LAN, there may be limitations in the height, length, area and the like of the antenna due to the size of the wireless LAN antenna.
In detail, the radiation electrode 13 of the antenna must be positioned farthest from a ground surface of a Printed Circuit Board (PCB) and the area thereof must be large so as to allow the antenna having the structure of FIG. 1 to have a suitable center frequency and to realize required impedance matching. However, most wireless LAN products recently developed are formed in a card shape, like a Personal Computer Memory Card International Association (PCMCIA) card and a Compact Flash (CF) card. Therefore, a maximum height between the radiation electrode and the ground surface of the antenna is limited.
Therefore, in the case of a dual band wireless LAN antenna, satisfactory transmission/reception characteristics cannot be obtained in 2.4 GHz and 5 GHz frequency bands due to the limitations of the height and area of the antenna.
FIG. 2 is a graph showing the characteristics of a dual band wireless LAN antenna for 2.4 GHz/5 GHz frequency bands, implemented using the conventional structure.
Referring to the graph of FIG. 2, it can be seen that a Voltage Standing Wave Ratio (VSWR) curve forms valleys that have narrow widths and, thus, are sharp in the 2.4 GHz and 5 GHz frequency bands in the conventional dual band wireless LAN antenna. In terms of frequency bands between markers P1 and P2 and between markers P3 and P4, there is a problem in that, since VSWR values in the 2.4 GHz frequency band are greater than two, signal characteristics of the 2.4 GHz frequency band are degraded. In terms of signal characteristics, there is a problem in that, since a bandwidth in the 2.4 GHz frequency band satisfying a VSWR value equal to or less than two is narrow, antenna characteristics are easily deviated depending on the variation of sets or surrounding environments.
In order to solve the problems, the area of the radiation electrode must be widened or the distance between the radiation electrode and the ground must be increased, as described above. However, in this case, there is a problem in that the size of the antenna increases. Consequently, it is difficult to apply the antenna to the card-shaped wireless LAN products.